In an endless belt for the power transmission in which a plurality of elements are supported by a looped band-shaped member, which belt is wound around a pair of variable pulleys, these elements each comprise a main body section that undergoes pinching pressure from the pair of variable pulleys, and a head section connected to the main body section via a neck section. A plurality of groove sections provided to facilitate removal of an oil film are formed on a side surface (flank surface) of the main body portion, which surface comes into contact with a belt sliding surface of the variable pulley (for example, refer to patent document 1 or patent document 2).
The main kinds of behavioral deterioration of the element are deterioration of yawing motion and deterioration of pitching behavior. Deterioration of the yawing motion, for example, partial hitting of the side surface (flank surface) of the element against the belt sliding surface of the variable pulley, has a great impact on transmission of a torque. Usually, at the entrance of the variable pulleys (at the upstream side of a belt traveling direction), in the case of the element having a deteriorated yawing motion, the yawing motion of the element is improved in such a manner that in a transition region of an idle arc (a region in which no compression force acts in between the elements) and an active arc (a region in which compression force acts in between the elements) in the variable pulley, a rear surface side of a preceding side surface (flank surface) of the element in the belt traveling direction slides against the variable pulley.
However, in the case of the element described in patent document 1, a groove portion provided to facilitate removal of an oil film is formed on the side surface (flank surface) of the element in a straight manner from the front surface to the rear surface of the element in the belt traveling direction. For this reason, the yawing motion of the element deteriorates at the entrance of the variable pulley, and even if the side surface (flank surface) of the element partially comes into contact with the belt sliding surface of the variable pulley, the action of removing an oil film by means of the groove portion does not change, and frictional force between the element and the variable pulley does not vary. In other words, when the element partially comes into contact with the belt sliding surface, it does not readily slide on the belt sliding surface, and thus, the element as above does not have an effective shape from the standpoint of improvement in the yawing motion thereof in the variable pulley.
In the element described in patent document 2, the intervals of groove portions provided to facilitate removal of an oil film are set, on the side surface (flank surface) of the element, such that distances between adjacent groove portions at the front surface side of the element in the belt traveling direction are wider and those at the rear surface side of the element in the belt traveling direction are narrower. For this reason, the yawing motion of the element deteriorates at the entrance of the variable pulley, and when the front surface side and rear surface side of the side surface (flank surface) of the element in the belt traveling direction each partially come into contact with the belt sliding surface, increase of surface pressure caused by a decrease of an area coming into contact with the variable pulley becomes an issue at the front surface side of the element in the belt traveling direction. Further, at the rear surface side in the belt traveling direction, increase of the area coming into contact with the variable pulley produces an effect of reducing a surface pressure, but the action of removing an oil film does not change, whereby reduction of frictional force between the element and the variable pulley cannot be expected. In other words, when this element partially comes into contact with the belt sliding surface, it also does not readily slide on the belt sliding surface, and therefore, the element does not have an effective shape from the standpoint of improvement in the yawing motion in the variable pulleys.
Patent document 1. Japanese Patent Application Laid-Open (JP-A) No. 2-236045
Patent document 2: Japanese Patent Application Laid-Open (JP-A) No. 10-213185